Notes on the Troubleshooting and Repair of Computer and Video Monitors


  6.4) Test equipment

Don't start with the electronic test equipment, start with some analytical
thinking.  Your powers of observation (and a little experience) will make
a good start.   Your built in senses and that stuff between
your ears represents the most important test equipment you have.

However, some test equipment will be needed:

* Multitester (DMM or VOM) - This is essential for checking of power supply
  voltages and voltages on the pins of ICs or other components - service
  literature like the SAMs Photofacts described elsewhere in this document
  include voltage measurements at nearly every circuit tie point for properly
  functioning equipment.  The multitester will also be used to check
  components like transistors, resistors, and capacitors for correct value
  and for shorts or opens.  You do not need a fancy instrument.  A basic
  DMM - as long as it is reliable - will suffice for most troubleshooting.
  If you want one that will last for many years, go with a Fluke.  However,
  even the mid range DMMs from Radio Shack have proven to be reliable and
  of acceptable accuracy.  For some kinds of measurements - to deduce trends
  for example - an analog VOM is preferred (though some DMMs have a bar graph
  scale which almost as good).

* Oscilloscope - While many problems can be dealt with using just a multimeter,
  a 'scope will be essential as you get more into advanced troubleshooting.
  Basic requirements are: dual trace, 10-20 MHz minimum vertical bandwidth,
  delayed sweep desirable but not essential.  A good set of proper 10x/1x
  probes.  Higher vertical bandwidth is desirable but most consumer electronics
  work can be done with a 10 MHz scope.  A storage scope or digital scope
  might be desirable for certain tasks but is by no means essential for basic

  I would recommend a good used Tektronix or HP scope over a new
  scope of almost any other brand.  You will usually get more scope
  for your money and these things last almost forever.  My 'good' scope
  is the militarized version (AN/USM-281A) of the HP180 lab scope.  This
  has a dual channel 50 MHz vertical plugin and a delayed sweep horizontal
  plugin.  I have seen these going for under $300 from surplus outfits.
  For a little more money, you can get a Tek 465 100 Mhz scope ($400-700)
  which will suffice for all but the most demanding (read: RF or high
  speed digital) repairs.

* A video signal source - depending on what type of monitor you are repairing,
  you may need both computer and television signals.

  Computer Monitors - a test PC is useful as a video source.  Of course,
  it will need to support whatever scan rates and video types the
  monitor is designed to accept.  Software programs are available to
  display purity, convergence, focus, color, and other test patterns.
  Or create your own test patterns using a program like Windows Paint.
  See the section: "Using a PC as a monitor test pattern generator".

  Studio monitors - a baseband video source like a VCR or camcorder
  is useful in lieu of a test pattern generator.  These will allow you to
  you to control the program material.  In fact, making some test tapes
  using a camcorder or video camera to record static test patterns will
  allow you full control of what is being displayed and for how long.

* Color bar/dot/crosshatch signal generator.  This is a useful piece
  of equipment if you are doing a lot of TV or studio monitor repair and
  need to perform CRT convergence and chroma adjustments.  However, there
  are alternatives that are almost as good: a VHS recording of these
  test patterns will work for TVs.  A PC programmed to output a suitable
  set of test patterns will be fine for monitors (and TVs if you can set
  up the video card to produce an NTSC/PAL signal.  This can be put
  through a VCR to generate the RF (Channel 3/4) input to your TV if
  it does not have direct video inputs (RCA jacks).

  Sophisticated (and expensive) universal test pattern generators are available
  that will handle any possible monitor scan rate.

  6.5) Incredibly handy widgets

These are the little gadgets and homemade testers that are useful for many
repair situations.  Here are just a few of the most basic:

* Series light bulb for current limiting during the testing of TVs,
  monitors, switching power supplies, audio power amplifiers, etc. I built
  a dual outlet box with the outlets wired in series so that a lamp
  can be plugged into one outlet and the device under test into the other.
  For added versatility, add a regular outlet and 'kill' switch using a
  quad box instead.  The use of a series load will prevent your expensive
  replacement part like a horizontal output transistor from blowing if
  there is still some fault in the circuit you have failed to locate.

* A Variac.  It doesn't need to be large - a 2 A Variac mounted with
  a switch, outlet and fuse will suffice for most tasks.  However,
  a 5 amp or larger Variac is desirable.  If you will be troubleshooting
  220 VAC equipment in the US, there are Variacs that will output 0-240 VAC
  from a 115 VAC line (just make sure you don't forget that this can easily
  fry your 115 VAC equipment.)  By varying the line voltage, not only can
  you bring up a newly repaired monitor gradually to make sure there are no
  problems; you can also evaluate behavior at low and high line voltage.
  This can greatly aid in troubleshooting power supply problems.  Warning:
  a Variac is not an isolation transformer and does not help with respect
  to safety.  You need an isolation transformer as well.

* Isolation transformer.  This is very important for safely working on
  live chassis equipment.  Since nearly all modern monitors utilize line
  connected switchmode power supply or line connected deflection circuits,
  it is essential.  You can build one from a pair of similar
  power transformers back-to-back (with their highest rated secondaries
  connected together.  I built mine from a couple of similar old
  tube type TV power transformers mounted on a board with an outlet box
  including a fuse.  Their high voltage windings were connected together.
  The unused low voltage windings can be put in series with the primary
  or output windings to adjust voltage.  Alternatively, commercial line
  isolation transformers suitable for TV troubleshooting are available
  for less than $100 - well worth every penny.

* Variable isolation transformer.  You don't need to buy a fancy combination
  unit.  A Variac can be followed by a normal isolation transformer.  (The
  opposite order also works.  There may be some subtle differences in
  load capacity.).

* Degaussing coil.  Make or buy.  The internal degaussing coil salvaged
  from a defunct color TV or monitor doubled over to half it original diameter
  to increase its strength in series with a 200 W light bulb for current
  limiting will work just fine.  Or, buy one from a place like MCM Electronics
  for about $15-$30 that will be suitable for all but the largest TVs and
  monitors.   Also, see the section:  "Degaussing (demagnetizing) a CRT".

  6.6) Safe discharging of capacitors in TVs and video monitors

It is essential - for your safety and to prevent damage to the device under
test as well as your test equipment - that large or high voltage capacitors
be fully discharged before measurements are made, soldering is attempted,
or the circuitry is touched in any way.  Some of the large filter capacitors
commonly found in line operated equipment store a potentially lethal charge.

This doesn't mean that every one of the 250 capacitors in your TV need to be
discharged every time you power off and want to make a measurement.  However,
the large main filter capacitors and other capacitors in the power supplies
should be checked and discharged if any significant voltage is found after
powering off (or before any testing - some capacitors (like the high voltage
of the CRT in a TV or video monitor) will retain a dangerous or at least
painful charge for days or longer!)

The technique I recommend is to use a high wattage resistor of about
100 ohms/V of the working voltage of the capacitor.  This will
prevent the arc-welding associated with screwdriver discharge but will
have a short enough time constant so that the capacitor will drop to
a low voltage in at most a few seconds (dependent of course on the
RC time constant and its original voltage).

Then check with a voltmeter to be double sure.  Better yet, monitor
while discharging (not needed for the CRT - discharge is nearly
instantaneous even with multi-M ohm resistor).

Obviously, make sure that you are well insulated!

* For the main capacitors in a switching power supply which might be
  100 uF at 350 V this would mean a 5K 10W resistor.  RC=.5 second.
  5RC=2.5 seconds.  A lower wattage resistor can be used since the total
  energy in not that great.   If you want to be more high tech, you can
  build the capacitor discharge circuit outlined in the companion
  document: "Testing capacitors with a multimeter and safe discharge".
  This provides a visible indication of remaining charge and polarity.

* For the CRT, use a high wattage (not for power but to hold off the high
  voltage which could jump across a tiny 1/4 watt job) resistor of a few
  M ohms discharged to the chassis ground connected to the outside of the
  CRT - NOT SIGNAL GROUND ON THE MAIN BOARD as you may damage sensitive
  circuitry.  The time constant is very short - a ms or so.  However, repeat
  a few times to be sure.  (Using a shorting clip lead may not be a bad idea
  as well while working on the equipment - there have been too many stories
  of painful experiences from charge developing for whatever reasons ready
  to bite when the HV lead is reconnected.)  Note that if you are touching the
  little board on the neck of the CRT, you may want to discharge the HV
  even if you are not disconnecting the fat red wire - the focus and screen
  (G2) voltages on that board are derived from the CRT HV.

  WARNING: Most common resistors - even 5 W jobs - are rated for only a few
  hundred volts and are not suitable for the 25KV or more found in modern
  TVs and monitors.  Alternatives to a long string of regular resistors are
  a high voltage probe or a known good focus/screen divider network.  However,
  note that the discharge time constant with these may be a few seconds.  Also
  see the section: "Additional information on discharging CRTs".

  If you are not going to be removing the CRT anode connection, replacing
  the flyback, or going near the components on the little board on the neck
  of the CRT, I would just stay away from the fat red wire and what it is
  connected to including the focus and screen wires.  Repeatedly shoving
  a screwdriver under the anode cap risks scratching the CRT envelope which
  is something you really do not want to do.

Again, always double check with a reliable voltmeter!

Reasons to use a resistor and not a screwdriver to discharge capacitors:

1. It will not destroy screwdrivers and capacitor terminals.

2. It will not damage the capacitor (due to the current pulse).

3. It will reduce your spouse's stress level in not having to hear those
   scary snaps and crackles.

  6.7) Additional information on discharging CRTs

You may hear that it is only safe to discharge from the Ultor to the Dag.
So, what the @#$% are they talking about? :-).

(From: Asimov (mike.ross@juxta.mnet.pubnix.ten)).

'Dag' is short for Aquadag. It is a type of paint made of a graphite pigment
which is conductive. It is painted onto the inside and outside of picture
tubes to form the 2 plates of a high voltage filter capacitor using the glass
in between as dielectric. This capacitor is between .005uF and .01uF in
value. This seems like very little capacity but it can store a substantial
charge with 25,000 volts applied.

The outside "dag" is always connected to the circuit chassis ground via a
series of springs, clips, and wires around the picture tube. The high voltage
or "Ultor" terminal must be discharged to chassis ground before working on the
circuit especially with older TV's which didn't use a voltage divider to
derive the focus potential or newer TV's with a defective open divider.

For more details, see the document: "TV and Monitor CRT (Picture Tube)

  6.8) The series light bulb trick

When powering up a monitor (or any other modern electronic devices with
expensive power semiconductors) that has had work done on any power circuits,
it is desirable to minimize the chance of blowing your newly installed parts
should there still be a fault.  There are two ways of doing this: use of a
Variac to bring up the AC line voltage gradually and the use of a series load
to limit current to power semiconductors.

Actually using a series load - a light bulb is just a readily available
cheap load - is better than a Variac (well both might be better still) since
it will limit current to (hopefully) non-destructive levels.

What you want to do is limit current to the critical parts - usually the
horizontal output transistor (HOT).  Most of the time you will get away with
putting it in series with the AC line.  However, sometimes, putting a light
bulb directly in the B+ circuit will provide better protection as it will
limit the current out of the main filter capacitors to the HOT.  Actually,
an actual power resistor is probably better as its resistance is constant
as opposed to a light bulb which will vary by 1:10 from cold to hot.  The
light bulb, however, provides a nice visual indication of the current drawn
by the circuit under test.  For example:

* Full brightness: short circuit or extremely heavy load - a fault probably
  is still present.

* Initially bright but then settles at reduced brightness: filter capacitors
  charge, then lower current to rest of circuit.  This is what is expected
  when the equipment is operating normally.  There could still be a problem
  with the power circuits but it will probably not result in an immediate
  catastrophic failure.

* Pulsating: power supply is trying to come up but shutting down due to
  overcurrent or overvoltage condition.  This could be due to a continuing
  fault or the light bulb may be too small for the equipment.

Note: for a TV or monitor, it may be necessary (and desirable) to unplug the
degauss coil as this represents a heavy initial load which may prevent the unit
from starting up with the light bulb in the circuit.

The following are suggested starting wattages:

* 40 W bulb for VCR or laptop computer switching power supplies.
* 100 W bulb for small (i.e., B/W or 13 inch color) monitors or TVs.
* 150-200 W bulb for large color monitors or projection TVs.

A 50/100/150 W (or similar) 3-way bulb in an appropriate socket comes in
handy for this but mark the switch so that you know which setting is which!

Depending on the power rating of the equipment, these wattages may need to be
increased.  I have had to go to a 300 W light bulb for some computer monitors.
However, start low.  If the bulb lights at full brightness, you know there is
still a major fault.  If it flickers or the TV (or other device) does not quite
come fully up, then it should be safe to go to a larger bulb.  Resist the
temptation to immediately remove the bulb at this point - I have been screwed
by doing this.  Try a larger one first.  The behavior should improve.  If it
does not, there is still a fault present.

Note that some TVs and monitors simply will not power up at all with any kind
of series load - at least not with one small enough (in terms of wattage) to
provide any real protection.  The microcontroller apparently senses the drop
in voltage and shuts the unit down or continuously cycles power.  Fortunately,
these seem to be the exceptions.

  6.9) Getting inside a monitor

You will void the warranty - at least in principle.  There are usually no
warranty seals on a monitor so unless you cause visible damage or mangle the
screws or plastic, it is unlikely that this would be detected.  You need to
decide.  A monitor still under warranty should probably be returned for
warranty service for any covered problems except those with the most obvious
and easy solutions.  Another advantage of using warranty service is that
should your problem actually be covered by a design change, this will be
performed free of charge.  And, you cannot generally fix a problem which
is due to poor design!

Getting into a monitor is usually quite simple requiring the removal of 2-10
Philips or 1/4" hex head screws - most around the edge of the cabinet or
underneath, a couple perhaps in the rear.  Disconnect the input and power
cables first as it they stay with catch on the rear cover you are detaching.
Reconnect whatever is needed for testing after the cover is removed.  Set
the screws aside and make notes if they are not all of the same length
and thread type - putting a too long screw in the wrong place can short out
a circuit board or break something else, for example.  A screw that is
too short may not be secure.

Once all visible screws are out, try to remove the cover.  There still
may be hidden catches or snaps around the edges or seam or hidden beneath
little plastic or rubber cosmetic covers.  Sometimes, the tilt-swivel base
will need to be removed first.  If no snaps or catches are in evidence,
the cover may just need a bit of persuasion in the form of a carefully
placed screwdriver blade (but be careful not to damage the soft plastic).
A 'splitting' tool is actually sold for this purpose.

As you pull the cover straight back (usually) and off, make sure that no
other wires are still attached.  Often, the main circuit board rests on
the bottom of the cover in some slots.  Go slow as this circuit board may
try to come along with the back.  Once the back is off, you may need to prop
the circuit board up with a block of wood to prevent stress damage and contact
with the work surface.

Most - but not all - monitors can be safely and stably positioned either
still on the tilt-swivel base or on the bottom of the frame.  However, some
will require care as the circuit board will be vulnerable.

Larger monitors are quite heavy and bulky.  Get someone to help and take
precautions if yours is one of the unstable variety.  If need be, the monitor
can usually safely be positioned on the CRT face if it is supported by
foam or a folded blanket.

Once the cover is off, you will find anywhere from none to a frustratingly
large number of sheetmetal (perforated or solid) shields.  Depending on which
circuit boards need to be accessed, one or more of these shields may need
to be removed.  Make notes of which screws go where and store in a safe
place.  However, manufacturers often place holes at strategic locations
in order to access adjustments - check for these before going to a lot
of unnecessary bother.  Note: sheetmetal usually has sharp edges.  Take care.

Reassemble in reverse order.  Getting the circuit board to slide smoothly
into its slots may take a couple of attempts but otherwise there should
be no surprises.

  6.10) Specific considerations before poking around inside a TV or monitor

Both electrical and mechanical dangers lurk:

* Main filter capacitor(s).  This is the most dangerous (not the HV as you
  would expect).  Fortunately, these capacitors will normally discharge in
  a few minutes or less especially if the unit is basically working as the
  load will normally discharge the capacitors nearly fully as power is
  turned off.  With TVs, the main filter capacitor is nearly always on the
  mainboard.  Monitors are more likely to have a separate power supply

  However, you should check across this capacitor - usually only one and by
  far the largest in the unit - with a voltmeter and discharge as suggested
  in the section: "Safe discharging of capacitors in TVs and video monitors"
  if it holds more than a few volts (or wait longer) before touching anything.

  Some of these are as large as 1,000 uF charged to 160 V - about 13 w-s or
  a similar amount of energy as that stored in an electronic flash.  This is
  enough to be potentially lethal under the wrong circumstances.
* High Voltage capacitor formed by the envelope of the CRT.  It is connected
  to the flyback transformer by the fat (usually red) wire at the suction cup
  (well, it looks like one anyhow) attached to the CRT.  This capacitor can
  hold a charge for quite a while - weeks in the case of an old tube type TV!

  If you want to be doubly sure, discharge this also.  However, unless you
  are going to be removing the HV connector/flyback, it should not bother you.

  The energy stored is about 1 w-s but if you touch it or come near to an
  exposed terminal, due to the high voltage, you will likely be handed *all*
  the energy and you *will* feel it.  The danger is probably more in the
  collateral damage when you jump ripping flesh and smashing your head against
  the ceiling.

  Some people calibrate their jump based on voltage - about 1 inch/V. :-).

  There will be some HV on the back of the circuit board on the neck of the
  CRT but although you might receive a tingle but accidentally touching the
  focus or screen (G2) pins, it is not likely to be dangerous.

* CRT implosion risk.  Don't hammer on it.  However, it is more likely that
  you will break the neck off the tube since the neck is relatively weak.  This
  will ruin your whole day and the TV or monitor but will likely not result in
  flying glass everywhere.  Just, don't go out of your way to find out.

* Sharp sheet metal and so forth.  This is not in itself dangerous but
  a reflex reaction can send your flesh into it with nasty consequences.

  6.11) Dusting out the inside of a monitor

The first thing you will notice when you remove the cover is how super
dusty everything is.  Complements to the maid.  You never dreamed there
was that much dust, dirt, and grime, in the entire house or office building!

Use a soft brush (like a new paintbrush) and a vacuum cleaner to carefully
remove the built up dust.  Blowing off the dust will likely not hurt the unit
unless it gets redeposited inside various controls or switches but will
be bad for your lungs - and will spread dirt all over the room.  Don't turn
anything - many critical adjustments masquerade as screws that just beg to
be tightened.  Resist the impulse for being neat and tidy until you know
exactly what you are doing.  Be especially careful around the components on
the neck of the CRT - picture tube - as some of these are easily shifted
in position and control the most dreaded of adjustments - for color purity
and convergence.  In particular, there will be a series of adjustable ring
magnets.  It is a good idea to mark their position in any case with
some white paint, 'white out', or a Magic Marker so that if they do get
moved - or you move them deliberately, you will know where you started.

  6.12) Troubleshooting a monitor with the mainboard disconnected

There are times when it is desirable to remove the chassis or mainboard and
work on it in a convenient location without having to worry about the
attachments to the CRT and cabinet circuitry.

My approach is usually to do as much work as possible without removing the
main board and not attempt to power it up when disconnected since there are
too many unknowns.  Professionals will plug the chassis into a piece of
equipment which will simulate the critical functions but this is rarely
an option for the doit-yourselfer.

Note that if you have a failure of the power supply - blown fuse, startup,
etc., then it should be fine to disconnect the CRT since these problems
are usually totally unrelated.  Tests should be valid.

However, if you really want to do live testing with the main board removed,
here are some considerations.  There are usually several connections to the
CRT and cabinet:

* Deflection yoke - since the horizontal coils are part of the horizontal
  flyback circuit, there could be problems running without a yoke.  This
  could be anything from it appearing totally dead to an overheating or
  blown horizontal output transistor.  There may be no problems.  Vertical
  and any convergence coils may or may not be problems as well.

* CRT video Driver board - pulling this should not usually affect anything
  except possibly video output and bias voltages.

* CRT 2nd anode - without the CRT, there will be no capacitor to filter
  the high voltage and you would certaily want to insulate the HV connector
  **real** well.  I do not know whether there are cases where damage to
  flyback could result from running in thie manner, however.

* Front panel controls - disconnecting these may result in inability to
  even turn the unit on, erratic operation, and other unexpected behavior.

* Degauss - you just won't have this function when disconnected.  But who
  cares - you are not going to be looking at the screen anyhow.

* Remote sensor - no remote control but I doubt that the floating
  signals will cause problems.

* Speakers - there will be no audio but this should not cause damage.

If you do disconnect everything, make sure to label any connectors whose
location or orientation may be ambiguous.  Most of the time, these will
only fit one way but not always.

Chapter 7) Monitor Adjustments

  7.1) User picture adjustment

For general viewing, subdued lighting is preferred.  Avoid backlighting
and direct overhead lighting if possible.

Display an image with a variety of colors and the full range of brightness
from deep shadows to strong highlights.  For PCs, a Windows desktop is
generally satisfactory.  An outdoor scene on a sunny day is excellent for
studio monitors.  Alternatively, use a test pattern specially designed
for this purpose.

Turn the BRIGHTNESS and CONTRAST controls (or use the buttons) all the way

Increase the BRIGHTNESS until a raster is just visible in the darkest
(shadow) areas of the picture.

Increase the CONTRAST until the desired intensity of highlights is obtained.

Since BRIGHTNESS and CONTRAST are not always independent, go back and forth
until you get the best picture.

On monitors with a color balance adjustment, you may want to set this but
unless you are doing photorealistic work, using the manufacturer's defaults
will be fine unless you need to match the characteristics of multiple
monitors located side-by-side.

  7.2) Focus adjustment

One of the most common complaints is that the monitor is not as crisp as
it used to be - or just not as sharp as expected.

Assuming that the focus has just been gradually getting worse over time,
tweaking the internal focus control may be all that is needed.

Some monitors have the focus adjustment accessible through a (possibly
unmarked) hole in the side or rear of the case.  If there is a single
hole, it is almost certainly for overall focus.  If there are two holes,
one may be the screen (G2 - master brightness) or the two adjustments may
be for different aspects of focus (e.g., horizontal and vertical).  Just
carefully observe what happens when each adjustment is moved a little so
that you can return it to its original setting if you turned the wrong one.
Use a thin insulated screwdriver - preferably with a plastic blade.  As
a extra precaution, determine of the screwdriver will mate easily with the
adjustment with the monitor **off** (don't turn anything, however).

Where there are two adjustment knobs on the flyback transformer, the top one
is generally for focus and the bottom one is for G2.

Most inexpensive monitors have only what is known as static focus - a constant
voltage derived from the HV power supply is applied to the focus grid of the
CRT.  This does not allow for optimal focus across the screen and any setting
is just a compromise between central and edge sharpness.

Better monitors will have (in addition) H and V focus controls.  These are
for dynamic focus adjustments.  There may be some interaction between the
static and dynamic adjustments.  If either of these controls has no effect or
insufficient range, then there may be a fault in the circuitry for that
particular adjustment -  a fault with the driver, waveform source, power
supply, etc.  The most sophisticated schemes use a microprocessor (or at least
digital logic) to specify the waveform for each section of the screen with a
map of correction values stored in non-volatile memory.  It would be virtually
impossible to troubleshoot these systems without detailed service information
and an oscilloscope - and even then you might need a custom adapter cable and
PC software to adjust values!

Also see the section: "About the quality of monitor focus".

If you need to go inside to tweak focus pots:
Safety: as long as you do not go near anything else inside the monitor while
it is on AND keep one hand in you pocket, you should be able to do this without
a shocking experience.

Plug it in, turn it on and let it warm up for a half hour or so.  Set your
PC (or other video source) to display in the resolution you use most often.
First turn the user brightness and contrast fully counterclockwise.  Turn
brightness up until the raster lines in a totally black area appear, then
back a hair until they disappear.  Then, turn the contrast control up until
you get a fairly bright picture.  Fullly clockwise is probably ok.  Adjust
FOCUS for generally best focus.  You will not be able to get it razor sharp
all over the screen - start at the center and then try to get the
edges and corners as good as you can without messing up the center too much.
Double check that the focus is ok at your normal settings of brightness and
contrast and at other resolutions that you normally use.

The focus pot is usually located on the flyback transformer or on an
auxiliary panel nearby.  The focus wire usually comes from the flyback or
the general area or from a terminal on a voltage the multiplier module
(if used).  It is usually a wire by itself going to the little board
on the neck of the CRT.

The SCREEN control adjusts background brightness.  If the two controls are
not marked, you will not do any damage by turning the wrong one - it will
be immediately obvious as the brightness will change rather than focus
and you can then return it to its original position (or refer to the section
on brightness adjustments to optimize its setting).

On a decent monitor, you should be able to make out the individual scanning
lines at all resolutions though it will be toughest at the highest scan rates.
If they lines are fuzzy, especially in bright areas, then focus may need
to be adjusted or there may be an actual fault in the focus circuitry or
a defective or just marginal CRT.

  7.3) Brightness and color balance adjustment

A monitor which has a picture that is very dark and cannot be adequately
set with the user brightness and contrast controls may need
internal adjustment of the SCREEN (the term, screen, here refers to a
particular electrode inside the CRT, not really the brightness of the screen
you see, though it applies here), MASTER BRIGHTNESS, or BACKGROUND level
controls.  As components age, including the CRT, the brightness will
change, usually decrease.  The following procedure will not rejuvenate
an old CRT but may get just enough brightness back to provide useful
functionality for a few months or longer.  If the problem is not with the age
of the CRT, then it may return the monitor to full brightness.  The assumption
here is that there is a picture but the dark areas are totally black and
the light areas are not bright enough even with the user brightness control
turned all the way up.

Note that circuit problems can also cause similar symptoms.  These are
particularly likely if the brightness descresed suddenly - CRT emission
problems will result in a gradual decrease in brightness over time.

In most cases, the cover will need to be removed.  The controls we
are looking for may be located in various places.  Rarely, there will
be access holes on the back or side.  However, if there are unmarked
holes, then the FOCUS and SCREEN controls are the most likely possibilities.

The controls may be located on the:

* Flyback (LOPT) transformer.  Usually there is a master screen control
  along with a focus control on the flyback transformer.

* A little board on the neck of the CRT.  There may be a master screen
  control. a master brightness control, a master background level control,
  or individual controls for red, green, and blue background level.  Other
  variations are possible.  There may also be individual gain/contrast

* Main video board is less common, but the background level controls may
  be located here.

Display a picture at the video resolution you consider most important
which includes both totally black and full white areas which also
includes sharp vertical edges.

Set the user brightness control to its midpoint and the user contrast
control as low as it will go - counterclockwise.

Let the monitor warm up for at least 15 minutes so that components can

If there is a MASTER BRIGHTNESS or BACKGROUND level control, use this to
make the black areas of the picture just barely disappear.  Them, increase
it until the raster lines just appear.  (They should be a neutral gray.
If there is a color tint, then the individual color background controls will
need to be adjusted to obtain a neutral gray.)  If there is no
such control, use the master screen control on the flyback.  If it is unmarked,
then try both of the controls on the flyback - one will be the screen control
and the other will be focus - the effects will be obvious.  If you did touch
focus, set it for best overall focus and then get back to the section on focus
once you are done here.

If there are individual controls for each color, you may use these but be
careful as you will be effecting the color balance.  Adjust so that the
raster lines in a black area are just visible and dark neutral gray.

If there is a 'service switch' you may prefer to make the adjustment
with this in the service position.  The raster will collapse to a single
horizontal line and the video input will be disabled and forced to black.
The BACKGROUND or SCREEN control can then be adjusted as above.

Now for the gain controls.   On the little board on the neck of the CRT
or on the video or main board there will be controls for R, G, and B DRIVE
(also may be called GAIN, or CONTRAST - they are the same).  The knobs or
slots may even be color coded as to which primary (R,G,B) it affects.
If there are only two then the third color is fixed and if the color balance
in the highlights of the picture was ok, then there is nothing more you can
do here.

Set the user contrast control as high as it will go - clockwise.

Now adjust each internal color DRIVE control as high as you can without
that particular color 'blooming' at very bright vertical edges.  Blooming
means that the focus deteriorates for that color and you get a big blotch
of color trailing off to the right of the edge.  You may need to go back
and forth among the 3 DRIVE controls since the color that blooms first
will limit the amount that you can increase the contrast settings.  Set
them so that you get the brightest neutral whites possible without any
single color blooming.

Note that this is ignoring the effects of any beam current or brightness
limiter circuitry.  Any recommendations in the service manual should be
followed to minimize the chance of excess X-ray emissions as well as to
avoid burn-in of the phosphor screen.

Now check out the range of the user controls and adjust the appropriate
internal controls where necessary.  You may need to touch up the background
levels or other settings.  Check at the other resolutions and refresh rates
that you normally use.

If none of this provides acceptable brightness, then either your CRT
is in its twilight years or there is something actually broken in the
monitor.  If the decrease in brightness has been a gradual process over the
course of years, then it is most likely the CRT.  As a last resort you can
try increasing the filament current to the CRT the way CRT boosters that
used to be sold for TVs worked.  See the section: "Brightening an old CRT".

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Written by Samuel M. Goldwasser. | [mailto]. The most recent version is available on the WWW server http://www.repairfaq.org/ [Copyright] [Disclaimer]